A λ/4 retardation film enables conversion of linearly polarized light to circularly polarized light or elliptically polarized light, or conversion of circularly polarized light or elliptically polarized light to linearly polarized light. Such a λ/4 retardation film is used in a wide variety of optical applications including image display devices, optical pickup apparatuses and the like.
A λ/4 retardation film used in an optical device which utilizes a laser light source having a particular wavelength, such as an optical pickup apparatus, will be acceptable if the λ/4 retardation film is capable of providing a phase difference (hereinafter, referred to as “retardation”) of ¼ wavelength only to light having a particular wavelength. However, a λ/4 retardation film that is used in a color image display device, an antireflective film for visible light and the like is required to be able to provide a retardation of ¼ wavelength to light over the entire visible light region.
Therefore, a λ/4 retardation film used in a color image display device is required to have negative wavelength dispersibility (reverse wavelength dispersibility), in which the retardation given to light having longer wavelengths is larger than the retardation given to light having shorter wavelengths. However, it has been difficult to obtain a film which gives a retardation of ¼ wavelength and exhibits sufficient negative wavelength dispersibility (reverse wavelength dispersibility); and particularly, a film which is single-layered and is capable of providing a retardation of λ/4 to light over a wide wavelength region.
A conventional λ/4 retardation film that exhibits negative wavelength dispersibility (reverse wavelength dispersibility) is obtained by bonding retardation films having different wavelength dispersing characteristics such that the optical axes thereof intersect each other (see, for example, PTL 1).
However, it has been difficult to produce such a film because it is necessary to adjust the retardation of each of the laminated retardation films to ¼ wavelength or ½ wavelength. Furthermore, since the ranges of retardation of the laminated retardation films are limited, the extent to which the wavelength dispersibility can be adjusted is limited, and thus a retardation of ¼ wavelength cannot be provided to light having a sufficiently wide wavelength region. Furthermore, in order to develop a retardation of ¼ wavelength or ½ wavelength in each film, it is necessary to increase the thickness of each film, resulting in the image display device having increased thickness.
As a method of reducing film thickness, a method is available wherein a material having high retardation development properties is added in the film. However, although a film containing a material having high retardation development properties can reduce the film thickness, the retardation in the in-plane direction or retardation in the thickness direction is prone to vary greatly as a result of slight variation in the thickness of the film. Therefore, there has been a problem that not only it is difficult to adjust the retardation, but also color unevenness or a decrease in contrast may occur.
PTL 2 proposes a retardation plate having optically anisotropic layer A having a retardation of λ/2 wavelength and optically anisotropic layer B having a retardation of λ/4 wavelength, wherein any one of the optically anisotropic layers A and B is a layer formed from liquid crystalline molecules. However, even for this retardation plate, since the retardations of the respective optically anisotropic layers are limited to ¼ wavelength or ½ wavelength as described above, it has been difficult to sufficiently control the wavelength dispersibility of the retardation plate.
In this regard, an investigation has been conducted on a retardation film which, even with a single layer, can achieve a good balance between the retardation and the wavelength dispersibility by the selection of a resin or a retardation controlling agent. For example, PTL 3 proposes a stretched film formed of a cellulose acetate having a particular degree of acetylation. PTL 4 proposes an optical film containing a compound having a coordinatability-imparting group and a birefringence-imparting group as a retardation controlling agent; PTL 5 proposes a retardation plate containing a cellulose ester and a retardation enhancer having two or more aromatic rings; and PTL 6 proposes an optical film containing, as a retardation controlling agent, a low molecular weight compound in which the magnitude of the dipole moment in a direction perpendicular to the major axis direction of the molecule is larger than the magnitude of the dipole moment in a direction parallel to the major axis direction of the molecule.